1. Field of the Invention
The present invention relates to a wavelength division multiplexing signal number monitoring apparatus and method for monitoring the number of signals in wavelength division multiplexing signal light in wavelength division multiplexing transmission systems.
2. Related Background Art
From social needs with the advent of highly technetronic society, there have been vigorous research and development concerning large-capacity, high-speed communications such as visual communications utilizing optical fiber transmission line networks and long-distance communications such as international communications. Here, wavelength division multiplexing (WDM) transmission systems, which perform high-speed/large-capacity optical communications by causing an optical fiber line to transmit therethrough a plurality of wavelengths of signal light (a plurality of signal light components having wavelengths different from each other), have been in the process of being developed and introduced as those responding to rapid increases in demands for communications due to the Internet and the like in recent years.
If the number of signals (number of channels) in signal light being transmitted changes, then the power of each signal light component may fluctuate in such a WDM transmission system because of transient fluctuations in amplification factor in an optical amplifier or the like. When the number of signal channels in the signal light being transmitted is constantly monitored, such a phenomenon of power fluctuation caused by changes in the signal number can be specified/detected so as to be distinguished from the changes in loss of the transmission line, whereby the power can be kept from fluctuating if the optical amplifier is controlled and so forth.
Known as such a signal number monitoring apparatus is one in which a demultiplexer/wavelength-branching device, such as an arrayed waveguide diffraction grating type optical demultiplexer, and light-receiving devices for detecting respective demultiplexed signal light components are combined together (see, e.g., the Institute of Electronics, Information and Communications Engineers, Communications Convention C-3-113, 1998).
FIG. 6 is a graph showing, in the case where wavelength division multiplexing signal light fed from a single input port of an arrayed waveguide diffraction grating type optical demultiplexer is branched/demultiplexed into a plurality of output ports corresponding to their respective predetermined output wavelengths, respective transmission characteristics (output wavelength characteristics) of five adjacent output ports from the (nxe2x88x922)-th to the (n+2)-th output ports. Here, the center wavelength in the transmission characteristic curve of each output port is the output wavelength thereof, whereas the output wavelength interval xcex94xcexo between the adjacent output ports is made to coincide with the signal wavelength interval xcex94xcexi (xcex94xcexi=0.4 nm in FIG. 6) of wavelength division multiplexing signal light.
Thus, wavelength division multiplexing signal light is inputted to the arrayed waveguide diffraction grating type optical demultiplexer in which the output wavelength of each output port corresponds to the signal wavelength of respective one signal light component, the demultiplexed signal light output is detected by a light-receiving device such as photodiode connected to the respective output port, and a detection number which is the number of photodiodes having detected signal light is counted, whereby the number of signals can be monitored constantly.
In an arrayed waveguide diffraction grating type optical multiplexer/demultiplexer, e.g., normal silica waveguide type demultiplexer, however, the transmission wavelength characteristic of each waveguide greatly depends on temperature, so that the transmission characteristic curve shifts upon changes in temperature, whereby the output center wavelength shifts, for example, on the order of dk/dT=0.1 nm/10xc2x0 C. This wavelength shift has a magnitude which is not negligible with respect to signal wavelength intervals of wavelength division multiplexing signal light. As a result, individual signal wavelengths of wavelength division multiplexing signal light and the respective output wavelengths outputted from output ports may lose their correspondence, so that, depending on the state of wavelength shift, for example, one channel of signal light may be outputted from two adjacent output ports, whereas two adjacent channels of signal light may be outputted from one output port, whereby the signal number cannot be counted accurately.
For example, if temperature rises by 20xc2x0 C. from the state where the output wavelength of the n-th output port is 1550 nm in FIG. 6, then the transmission wavelength characteristic of each output port generates a wavelength shift of 0.2 nm toward the longer wavelength side. At this time, the n-th output port yields an output wavelength of 1550.2 nm, whereas the (nxe2x88x921)-th output port yields an output wavelength of 1549.8 nm, whereby signal light having a signal wavelength of 1550 nm is outputted from both of these two output ports.
If the bandwidth of the transmission wavelength characteristic of each output port is set narrower in order to prevent one channel of signal light from being outputted from two output ports as such, then there will conversely be cases where no signal light is outputted from any output port, so that the signal number cannot be counted accurately.
If temperature control is carried out such that the temperature of the arrayed waveguide diffraction grating type demultiplexer is held constant, then the above-mentioned wavelength shift can be prevented, whereby the signal number can be counted accurately. Since the arrayed waveguide diffraction grating type optical demultiplexer is additionally provided with temperature control means in this case, however, it is problematic in that the apparatus increases its size and cost of manufacture, and so forth.
In view of the foregoing problems, it is an object of the present invention to provide a wavelength division multiplexing signal number monitoring apparatus and method which can accurately count the signal number independently of temperature.
The present invention provides a wavelength division multiplexing signal number monitoring apparatus for monitoring the number of signal light components (N at the maximum) included in wavelength division multiplexing signal light composed of a plurality of signal light components in which any two signal light components have a wavelength interval therebetween set to an integer multiple of xcex94xcexi. This apparatus comprises: (1) an arrayed waveguide diffraction grating type optical demultiplexer for guiding the wavelength division multiplexing signal light, demultiplexing the guided signal light at a wavelength interval of xcex94xcexo (where xcex94xcexo=xcex94xcexi/m, m being an integer of at least two), and outputting demultiplexed individual light components respectively from mxc3x97(N+l) output ports (where l is a predetermined integer of at least one); (2) a light-receiving device array comprising mxc3x97(N+l) light-receiving devices disposed so as to correspond to the respective output ports; and (3) a counter unit for receiving an output signal of each light-receiving device of the light-receiving device array and determining the number of signal light components included in the wavelength division multiplexing signal light according to the number of light-receiving devices which have detected light having a predetermined level or higher in each of m light-receiving device groups each combining (N+l) light-receiving devices together such that the light components to be detected have a wavelength interval of xcex94xcexi.
On the other hand, the present invention provides a wavelength division multiplexing signal number monitoring method for monitoring the number of signal light components (N at the maximum) included in the above-mentioned wavelength division multiplexing signal light, the method comprising the steps of demultiplexing the wavelength division multiplexing signal light at a wavelength interval of xcex94xcexo (where xcex94xcexo=xcex94xcexi/m, m being an integer of at least two); outputting demultiplexed individual light components respectively from mxc3x97(N+l) output ports (where l is an integer of at least one); detecting whether the light components outputted from the respective output ports have at least a predetermined level or not; counting the number of output ports each detected to have yielded a light component having at least the predetermined level in each of m output port groups each grouping (N+l) output ports such that the outputted light components have a wavelength interval of xcex94xcexi; and determining according to a result thereof the number of signal light components included in the wavelength division multiplexing signal light.
In accordance with the present invention, in each of the light-receiving device array groups (or output port groups), signal light is not detected from adjacent wavelength bands at intervals of m-th wavelength band among the divided wavelength bands. Even when the wavelength band of one signal light component extends over a plurality of wavelength bands, the possibility of these wavelength bands of signal light being outputted at the same time is limited to m output ports whose output wavelength bands are adjacent to each other among all the output ports, whereby the same signal light component would not be outputted nor detected from two output ports at the same time in each light-receiving device (output port) array group. Also, since the interval upon branching is shorter than the interval in signal light, two signal light components would not be outputted from the same output port. Therefore, signal light can accurately be counted independently of temperature fluctuations. Further, when a margin is provided (l is made greater) for the wavelength band, the signal light wavelength will not deviate from the demultiplexing region even if a temperature shift occurs, whereby reliable measurement can be carried out. Also, since it becomes unnecessary for the optical demultiplexer to be kept at a constant temperature, no temperature control apparatus is necessary, whereby the apparatus can be made smaller at a lower cost.
Preferably, the number of light-receiving devices (output ports) having detected light with a predetermined level or higher in each light-receiving device array group (output port group) is taken as a detection signal light number, and the maximum detection signal light number in all the light-receiving device array groups (output port groups) is determined as the number of signal light components included in the wavelength division multiplexing signal light.
Since the wavelength shifts accompanying temperature changes in each waveguide of an arrayed waveguide diffraction grating type optical demultiplexer occur in the same direction, output port groups or light-receiving device array groups in which the transmission center wavelength of each waveguide, i.e., the output center wavelength of the output port, coincides with the center wavelength of signal light are equivalent to those subjected to appropriate temperature correction, whereby an accurate detection signal light number is obtained. As the deviation between these wavelengths becomes greater, the output port groups or light-receiving device array groups are less likely to detect signal light, thus yielding a smaller detection signal light number. Hence, the maximum detection signal light number can be determined as the accurate signal light number. Here, though m can be made greater, setting m=2 is preferable since it is compatible with compactness in apparatus.
The output characteristic to each output port upon branching is preferably set such that the difference from the center wavelength at a wavelength shifted from the center wavelength by xc2x1xcex94xcexo/2 is smaller than 4 dB.
When the output characteristic is set as such, the overlap in transmission bands between output ports having wavelength bands adjacent to each other can be made sufficiently large. As a result, even in the case where signal light has a wavelength lying between the respective center wavelengths of output ports whose wavelength bands are adjacent to each other, it can be outputted with a sufficient level from any of the output ports.
The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.